In recent years, battery energy storage systems have been employed more and more in DC electric railroads (see non-patent literature 1 and 2 for instance). For example, by using a high-capacity lithium-ion battery mounted on a railcar, a travel distance of 25 [km] or more per charge can be achieved even when no power is supplied from contact wires.
In general, a voltage of contact wires for a DC electric railroad and an operating voltage of an energy accumulation element are different; therefore, voltage conversion (power conversion) is required using a bidirectional chopper circuit. FIG. 11 is a circuit diagram of a common bidirectional chopper circuit. A bidirectional chopper circuit 101 includes a first switching unit (positive-side valve device) 121-1 and a second switching unit (negative-side valve device) 121-2, which are connected in series with each other so that conduction directions in an ON state are aligned and one of which is turned OFF when the other is turned ON, and an inductor 113 connected to a connecting point of the first switching unit 121-1 and the second switching unit 121-2. Each of the first switching unit 121-1 and the second switching unit 121-2 consists of a semiconductor switching device that conducts in one direction when it is ON and a feedback diode connected in antiparallel with the semiconductor switching device. In a case of a DC electric railroad including a battery energy storage system, a high-voltage side DC voltage Vdc1 corresponds to a voltage of contact wires while a low-voltage side DC voltage Vdc2 corresponds to an operating voltage of an energy accumulation element. For example, when a standard voltage of contact wires is Vdc1=1500 [V], Vdc2 is set anywhere from 600 [V] to 700 [V]. In recent years, large-capacity bidirectional choppers with a converter capacity of 500 [kW] per single unit have been developed. In this case, a DC component of an inductor current iL is 500 [A] or higher.